The last battle of Brother-Captain Stern of the Grey Knights, given that he came closest to killing Abaddon. Although It becomes something of an Epic Fail when he removes his helmet for the killing blow and gets his head shot off by a humble guardsman
The defense of Titan, especially how they defeat the besieging army of Valchocht the Maker: they turn their litanies of hate into a Combined Energy Attack that wipes out every daemon on the surface. The sequel reveals that they keep fighting even 10,000 years later when all their bullets are spent, their blades are blunt, and their armor has stopped working.
They also manage to steal the Emperor's body from Valchocht, who had been using him as a host.
The defense of Nu Marsus against an invading Slaaneshi horde.
The Last Stand of the Orks against the New Devourer. To detail, Wazdakka Gutsmek, the greatest Ork Warboss left, leads the biggest Ork "WAAAAGGGHHHH!" in history with almost a million trillion Orks in the largest battle in the history of the entire galaxy. The account describes Ork ships ramming and then boarding New Devourer bioships, with the trillions of boyz' roars loud enough to shudder the air and melt brass.
As a side note the New Devourer. As mentioned above, literally the entire ork race throws itself against it. The result is an epic curb stomp battle that only ends when the orks are completely wiped out.
During Ethelred's battle against the Daemon Prince Lothar, a cunning Blood Claw defeats a Khorne Berserker guarding a bridge by running up beneath him and stabbing upwards to disable the Berserker, allowing a Rune Priest to decapitate the Chaos Warrior. Yes, that is a re-enactment of the battle against the Viking at Stamford Bridge.
A Baneblade manages to disable two Necron Monoliths (with a lucky shot, then with its machine spirit pulling a Heroic Sacrifice) so a transport can escape with all that remains of a world's population.
The galaxy is falling. The Emperor is dead and the absolute worst which could possibly happen has now happened. The Imperium is fragmented into petty wars and daemons run freely slaughtering and corrupting everything in their path. Space Marine chapters are turning from their noble duties, all but the Imperial Fists and their successors who carry out one final order by their chapter master - "Digin."